1. Field of the Invention
The present invention relates to a gain-clamped optical amplifier, and more particularly, to a gain-clamped optical amplifier using an all-optical method in an optical amplifier structure having a Raman cavity, without any loss of bandwidth.
2. Description of the Related Art
Much research has been conducted on fiber Raman amplifiers (FRAs), which are capable of amplifying optical signals having a wavelength band that cannot be amplified with an erbium-doped fiber amplifier (EDFA), and which are able to improve the performance of the EDFA.
Fiber Raman amplification has advantages in that an amplification bandwidth of an FRA can be continuously expanded by using multi-wavelength pumps and in that a flattening of a Raman gain band can be obtained by controlling the power of each multi-wavelength pump.
Research has been mainly conducted on FRAs of a distributed type, which compensate for a loss of an optical transmission line by pumping the transmission medium. Also, research is being conducted on FRAs of a discrete type similar to EDFAs to be generally used as a lumped type optical amplifier. Efforts have been made to replace EDFAs with FRAs due to the flexibility of the amplification bandwidth of the FRAs.
Recently, researches on FRA based on a laser cavity have been conducted, and their results have been shown its importance in aspects of miniaturization of FRAs by reducing a 10 km length of Raman medium to about 1 Km.
In general optical amplifiers, the optical gain depends on the intensity of signal light. This characteristic increases a bit error rate in an optical communication and deteriorates quality of data communication. Therefore, to solve this problem, a gain-clamped optical amplifier has been employed as a practical optical amplifier.
Two methods are being studied in typical gain clamping techniques. The first one is an all-optical method which passively clamps a gain in an optical way by using laser resonance, and the second one is a method which electrically controls a current of a pumping laser diode according to the intensity of incident light.
Since the second method requires a complex signal processing for gain clamping, much research has been conducted based on the first all-optical method. The all-optical gain clamping technique passively controls the amount of population inversion in a gain medium by generating a laser beam. That is, since a gain of an optical amplifier is proportional to the size of population inversion and the length of a gain medium, the gain of an optical amplifier is clamped according to the intensity of the oscillating laser counteracting the signal intensity.
If optical signals having a low power are introduced in all-optical gain clamped amplifier, power of the laser becomes automatically high so that the signal gain keeps a gain value. On the contrary, if power of the optical signals is high within a limited range, the power of the laser is somewhat reduced and the signal gain still has the same gain value regardless of the signal power. That is, since power competition between the optical signals and the laser is generated in a complementary relationship, a gain clamping of signals is achieved.
FIG. 1 is a schematic diagram illustrating a conventional gain-clamped EDFA having a laser oscillation structure. Referring to FIG. 1, an erbium-doped fiber (EDF) is used as a gain medium, and pumping light and signal light are combined by a wavelength division multiplexing (WDM) optical coupler. In order to obtain gain clamping, a laser cavity is configured with a ring cavity of a feedback loop type connecting an input terminal and an output terminal.
The ring cavity comprises an optical attenuator (ATT), an optical isolator (ISO), and a band pass filter (BPF), to control the laser oscillation.
The BPF sets a wavelength in which the laser oscillation occurs, and the ISO allows oscillation to occur in only one direction in a ring type resonator. This unidirectional oscillation prevents a hole-burning problem in a laser resonator (laser cavity).
The ATT controls an amplification gain of the signal light by controlling a laser power of the laser resonator.
FIG. 2 is a graph illustrating the principles of gain clamping occurring in a conventional gain-clamped optical amplifier structure. Referring to FIG. 2, when an optical signal is input to the gain-clamped optical amplifier, a gain change for a signal power does not occur since the laser power is automatically controlled with a high speed against variations of the signal power.
However, in this structure, since the oscillating laser is formed in an amplification band, a signal bandwidth allowing the gain clamped optical amplifier is reduced as much as a bandwidth of the laser.